On the PHPClasses.org blog Manuel Lemos has a recent post looking at a method for sending emails using a remote SMTP server (like Google or Yahoo's) via this package.

In most hosting environments, the PHP mail() function is sufficient to send e-mail messages without problems. However, there are situations on which using the mail function is inconvenient and may prevent that your messages reach the destination. [...] Therefore, you may need to find an alternative way for sending your e-mail messages from PHP.

The package lets you define a username/password, server, whether to use a SSL connection or not, and if the connection needs to use the TLS protocol.